The cross-validation correlations for the BIA determinations of % fat ranged from 0.71 to 0.76, which were significantly lower than that obtained with the sum of seven (sigma 7) skinfolds equations (rxy = 0.92 for men and 0.88 for women). The correlations between the weight-to-height ratio body mass index (BMI) and hydrostatically determined % fat were 0.75 and 0.74 for men and women, respectively. The standard errors of estimate for the two BIA models ranged from 4.6 to 6.4% fat compared with 2.6 and 3.6% fat for the sigma 7 equations. The BIA method for measuring body composition was comparable to the BMI method, with height and weight accounting for most of the variance in the BIA equation.

Ironically, that would indicate that your BMI is about as accurate as using one of these fancy impedance meters!

For those interested, here's another study by Girandola and Contarsy that found correlations of about 0.76. They conclude:

The results of the present study support the use of this bioelectrical impedance technique as simple, reliable and yet accurate method of assessing the percent of body fat in males and female in the clinical setting.

I guess it's a matter of opinions, but I would recommend a weight scale with a built in impedance meter, at least you'll get your weight from it!

And put the scale in front of a mirror.
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SparafusileMar 10 '11 at 0:34

I use a scale with built in BIA for hydration% and fat% calculations. I don't count on it to be extremely accurate, but I count on it to show me against it's previous readings which way I'm headed.
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Nathan Wheeler♦Mar 11 '11 at 18:34

True @md5sum, but I guess the point is that they fail to validly assess your body composition. If you were to loose fat but gain muscles, it might skew in another direction than when you only lost fat.
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Ivo Flipse♦Mar 11 '11 at 21:02

yeah, I just use it for a general idea without having to measure everything.
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Nathan Wheeler♦Mar 11 '11 at 21:04

please provide more information than just a link to Google.
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Ivo Flipse♦Mar 10 '11 at 13:09

I have no experience with such tools. How am I going to perform such check properly?
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gruszczyMar 10 '11 at 14:59

It's very easy. Measure the thickness of the skin of different parts of body with the caliper. In this website explain how to do it. linear-software.com/online.html The calculator return the bodyfat %. Measure every week at same hour.
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JoaquinGMar 11 '11 at 7:45

The site where the images come from have a guide for it @gruszcy, but you could ask it in a separate question
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Ivo Flipse♦Mar 11 '11 at 14:06

1

+1. The most important thing with measuring your progress is repeatability. This is something the impedance-based tools are not so good at as they can be thrown off by how well hydrated you are. But with skinfold measurements, as long you're using the tool properly every time, they should be very consistent. There's an overview of the different methods here... sparkpeople.com/resource/fitness_articles.asp?id=1045
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Steve WorthamDec 17 '11 at 15:13